The vast majority is plain language now following a national recommendation. Granby (Lost Acres) is one of the last holdouts that insists on signals and they don't use them all.
Off the top of my head, Hartford County Signals (other counties had some differences)
12 - refreshments
23 - emergency other than fire
24 - non emergency call
25 - storm warning
50 - fire
51 - report to your station
52 - off the air
53 - on the air
54 - incident under control
59 - phone call
60 - change channels
62 - mutual aid
65 - working fire
66 - refuel apparatus
71 - return to station
72 - chief at scene
73 - available for other assignment
75 - death in department
76 - search and rescue
83 - slow down
85 - radio test
87 - first apparatus on scene
88 - first officer on scene
99 - air crash
Some older outdated codes found here but some may still be current.
RADIO CODES & SIGNALS - CONNECTICUT
chris